| The ultra-hard material of AlMgB14has attracted wide attention in recent years because of its high hardness, excellent abrasive resistance, chemical inertness, low friction coefficient and cheap raw materials. It is a new member of hard materials after diamond, c-BN and B4C. AlMgB14based composites were mainly used in special machines in industries of military project, metal cutting, mining, forestry, petroleum, agriculture and so on, for its ability to stand wide temperature range. In recent, it has been shown that the micro-hardness of AlMgB14-TiB2composites could reach48GPa with excellent abrasive resistance, toughness and thermal conductivity. Field Activated and Pressure Assisted Synthesis (FAPAS) is a new technique for synthesizing materials under multiphysical coupling fields (electric, mechanical, temperature and chemical). With its advantages of high temperature, rapid heating speed, high efficiency and energy conservation, it has become a key technique for synthesizing high temperature resistant bulk functional materials.In this paper, the AlMgB14and its composites were synthesized by Mechanical Alloying(MA) and FAPAS process. The microstructure and component of synthesized metallic ceramic were observed and determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM). The synthesis mechanism, parameters, densification and mechanical properties of AlMgB,4-TiB2composites and AlMgB14-CNTs composites were investigated; the effect of temperature, pressure and other process parameters on the purity and performance of products were discussed; the characterization of the fracture toughness of synthetic cermets was evaluated using micro-hardness indentation method and the abrasive resistance of the sample surface was measured by abrasive wear experiment and single-point scratch test.The results show that, based on the reaction system of Al-Mg-B, dense AlMgB14bulk materials could be prepared using FAPAS, which offers a new way to synthesize AlMgB14. The phase diagram analysis of Al-Mg-B system shows that the sintering temperature above1000℃, the heating speed of100℃/min and sufficient sintering time is enough to get complete reaction and obtaine dense AlMgB14bulk materials.The process of preparing AlMgB,4bulk materials is furthered as follows: sintering temperature1400-1500℃, heating rate100℃/min, axial pressure60MPa, heat preservation8-10min, optimum starting powders ratio Al:Mg: B=0.1915:0.1363:0.6722, and the particle size at about2μm. As to the reaction system of Al-Mg-B, the AlMgB14materials prepared by FAPAS generally contain a small amount of MgAl2O4which comes from the reaction of Mg, Al with residual oxygen from ball mill grinding process. Adding excessive3wt.%Al and controlling temperature will be helpful to reduce the content of MgAl2O4. The average hardness of AlMgB14prepared under these conditions is27.2GPa, the average density is2.62g/cm3and the fracture toughness is3MPam1/2.The pre-reacted AlMgB14powders prepared by FAPAS are mixed with TiB2powers and then subjected to the temperature of1500℃, the axial pressure of60MPa, the heating speed of100℃/min and the heat preservation time of15minutes. AlMgB!4-TiB2composites with ideal structures are successfully prepared. Analysis results show that the micro-hardness (HV) of AlMgB14-TiB2 composites increases with the increasing the content of TiB2; when the content of TiB2is30%, the micro-hardness of the AlMgB14-TiB2composites is31.5GPa and the fracture toughness KIC could reach3.65MPa-1/2, which is20%higher than single AlMgB14. The microstructure analysis shows that the main toughening mechanisms of AlMgB14-TiB2composites are hard phase dispersion strengthening, high-strength interface bonding and the high elastic modulus and high-hardness toughening and strengthening of TiB2. Therefore, reducing the particle size of TiB2to nanoscale is an efficient way to improve the toughness and hardness.The results of friction and wear experiment at room temperature have shown that the addition of TiB2into AlMgB14enhances the abrasion-resistant property. The friction coefficient of single AlMgB,4ranges from0.45to0.55, while that of AlMgB14-TiB2composites ranges from0.4to0.45; under the same load, the abrasion loss of AlMgB,4-TiB2composites reduces with increasing the addition of TiB2; and the scratch width of AlMgB14-TiB2composites with the same composition increases with increasing pressure.The results of AlMgB14-CNTs composites prepared by FAPAS show that adding CNTs could both increase the hardness of AlMgB14and improve the fracture toughness. It was found that the addition of0.5%CNTs achieves the best reinforcing and toughening effect. The hardness and fracture toughness of AlMgB14-CNTs composite ceramic increase first and then decrease with increasing the content of CNTs, reaching maximum when the addition of CNTs accounts of0.5wt%and then decreasing gradually with increasing the content of CNTs. The microstructure analysis of composites with an addition of0.5%CNTs shows the mechanism as follows:the addition of CNTs refines the grain, promotes the formation of network microstructure and causes the crack deflection along the interface between CNTs and AlMgB14which leads to toughening. Thus, the high quality, uniform dispersion, integrity and compatibility with matrix of the CNTs are the factors that improve the fracture toughness of AlMgB14-CNTs composites dramatically; in addition, it was found from contrast experiments that AlMgB14-CNTs composites with the addition of0.5wt%CNTs achieve the best abrasion resistant property and possess certain self-lubricating property. |
